Glass-ceramic technology is founded in U.S. Pat. No. 2,920,971. That patent teaches the three general steps required in the manufacture of conventional glass-ceramic articles. Hence, a glass-forming batch, usually containing a nucleating agent, is first melted. The resulting melt is then simultaneously cooled to a substantially crystal-free glass and an article of a desired geometry shaped therefrom. Finally, the glass article is subjected to an explicitly-described heat treatment which causes the glass to crystallize in situ. As is also explained in that patent, the heat treatment promoting crystallization in situ is customarily conducted in two stages. First, the glass article is heated to a temperature somewhat above the transformation range of the glass to initiate the development of submicroscopic nuclei therein. Second, the nucleated glass is heated to a higher temperature, commonly above the softening point of the glass, to promote the growth of crystals on the nuclei.
Inasmuch as a glass-ceramic article is the result of the essentially simultaneous growth of crystals on countless nuclei dispersed throughout the parent glass body, the microstructure thereof comprises fine-grained crystals of relatively uniform size, homogeneously dispersed and randomly oriented within a residual glassy matrix. Glass-ceramic articles are normally very highly crystalline, i.e., considerably greater than 50% by volume crystalline. Because of that fact, the physical properties of such articles will be more closely akin to those exhibited by the crystal phase, rather than to those of the residual glassy matrix. Further, the residual glass will customarily have a very different composition from that of the parent glass since the constituents making up the crystal phase will have been removed therefrom.
For more information regarding the theoretical and practical considerations involved in the production of glass-ceramic articles, as well as for further discussion of the microstructure attendant in such articles, reference is hereby made to U.S. Pat. No. 2,920,971.
The widest use of glass-ceramic materials has been in the field of culinary ware. Cooking utensils have been marketed under the trademark CORNING WARE and flat sheeting for cooking surfaces on the top of stoves has been marketed under the trademark THE COUNTER THAT COOKS. Both types of products have been manufactured by Corning Glass Works, Corning N.Y.
It has been appreciated that compositions demonstrating good transmission of infra-red radiation would be very useful for culinary ware. Hence, the heat from the stove burner source would pass more quickly through the cross section of the ware and, thereby, expedite cooking. Also in the field of culinary ware, with particular emphasis on cooking vessels, market surveys have strongly indicated a consumer desire for transparent materials.
For use as culinary ware, a glass-ceramic material must be mechanically strong, have a low coefficient of thermal expansion, exhibit good chemical durability, and must be highly resistant to detergent attack and food staining. Furthermore, the parent glass must demonstrate the physical properties necessarily required for large scale melting and forming techniques. In sum, the final commercial product must not only display chemical and physical properties desirable in culinary applications, but must also be capable in the glass state of conforming to high speed production practices. It is with respect to these glass working characteristics that many of the proposed compositions for culinary ware have fallen short. Numerous problems have arisen such as, for example, very high melting temperatures have been required; the glass has been prone to devitrification; the glass viscosity has been such as to render it difficult to form and work; and firepolishing of the glass articles has been difficult at best.
The previously-marketed glass-ceramic materials for culinary use, such as the CORNING WARE and THE COUNTER THAT COOKS products noted above, have been opaque to visible radiation and very poorly transmitting to infra-red radiation. U.S. application Ser. No. 603,544, filed Aug. 11, 1975 by H. L. Rittler, discloses glass-ceramic articles which are opaque to visible light, but demonstrate relatively good transmittance to infra-red radiation. Thus, such articles will transmit up to about 60% of radiations having a wave length of 3.5 microns through a wall thickness of 4.25 mm. The compositions of those articles lie within a very narrowly-defined area of the Li.sub.2 O--ZnO--Al.sub.2 O.sub.3 --SiO.sub.2 quaternary, nucleated with TiO.sub.2, wherein beta-spodumene solid solution comprises the predominant crystal phase.
U.S. application Ser. No. 649,475, filed Jan. 15, 1976 by H. L. Rittler, describes the production of glass-ceramic articles which are transparent to visible radiation and highly transmitting in the infra-red portion of the spectrum. Thus, at thickness of 4 mm, such articles can transmit up to 80% of radiations having a wave length of 3.5 microns. The compositions disclosed therein are encompassed within an extremely narrow range of the Li.sub.2 O--Al.sub.2 O.sub.3 --SiO.sub.2 --P.sub.2 O.sub.5 quaternary, nucleated with a combination of TiO.sub.2 and ZrO.sub.2, wherein beta-quartz solid solution constitutes the predominant crystal phase. The presence of P.sub.2 O.sub.5 results in the replacement of some of the SiO.sub.2 in the beta-quartz structure with AlPO.sub.4.
Transparent glass-ceramic articles containing beta-quartz solid solution have been known to the prior art. Beta-quartz, the hexagonal trapezohedral modification of SiO.sub.2, exhibits very low birefringence, i.e., optical anisotropy, and a slightly negative coefficient of thermal expansion. This combination of properties has resulted in considerable research to develop practically commercial products from such bodies. The basis of the beta-quartz solid solution (also frequently termed beta-eucryptite solid solution) is believed to be the substitution of Al.sup.+.sup.3 ions for some of the Si.sup.+.sup.4 ions in the quartz structure, with the attendant charge deficiency being made up with the introduction of a small ion such as Li.sup.+, Mg.sup.+.sup.2, or Zn.sup.+.sup.2 into the quartz structure.